Zero sum gain in relationships means that when one person gains, the other person loses. When you put someone down, you might feel superior, but you've just pushed the other away: zero sum gain. If your church draws away attendees of other churches, your gain is the other churches loss: zero sum gain.
Giving compliments, accentuating the positive in situations, and being generous as a life-style is a win-win style where both sides are blessed and rise up.
In the parable of the talents, when the guy buried his talent, he felt secure that it was safe and would not be lost, but his master saw it as a loss because he did not put the talent given to use: zero sum gain.